Cardiac pacing devices deliver appropriately timed electrical stimulation pulses to a patient's heart to maintain a normal heart rhythm or improve synchronization of heart chambers. Patients having bradycardia, abnormalities of the heart's natural conduction system, or heart failure may benefit from artificial cardiac pacing of one or more heart chambers. In order to effectively pace the heart, an electrical impulse delivered to the heart must have sufficient energy to depolarize the myocardial cells. Depolarization of the myocardial cells in response to a pacing pulse is often referred to as “capture.” The cardiac electrogram signal evidencing capture, which may be a P-wave in the atria or an R-wave in the ventricles, is generally referred to as an “evoked response.” The lowest pacing pulse energy that captures the heart may be referred to as the “pacing threshold” or “capture threshold”. The amplitude and duration of a pacing pulse are preferably set to provide a pacing pulse energy somewhat greater than the pacing threshold in order to ensure effective cardiac pacing. However, in order to prolong the battery life of the implanted pacemaking device, it is desirable to program the pacing pulse energy to be a minimum value that is considered safely above the pacing threshold.
Pacing threshold however can change over time due to fibrotic encapsulation of the pacing electrodes, changes in the patient's clinical condition, changes in medical therapy, lead movement, or other causes. A rise in pacing threshold can result in loss of capture and ineffective pacing. Modern pacemakers, therefore, may include automatic pacing threshold search algorithms that automatically adjust the pacing pulse energy to ensure pacing pulses remain above the pacing threshold, even if it varies over time. A pacing threshold search may deliver pacing pulses starting at an initially high pulse energy that is greater than the pacing threshold and progressively decrease until capture is lost. The lowest pulse energy at which capture still occurs is determined as the pacing threshold. In order to reliably determine a pacing threshold, the cardiac pacing device must reliably discriminate between capture and loss of capture.
One method that has been implemented in commercially available devices for detecting capture is to sense the evoked response following a pacing pulse. Evoked response sensing may be to verify capture during pacing threshold searches and during normal cardiac pacing to ensure that effective pacing is provided. If a loss of capture is detected, as evidenced by the absence of an evoked response following a pacing pulse, a back-up pacing pulse of higher energy may be delivered and a pacing threshold search may be triggered to reset the pacing pulse energy.
Accurate capture verification and maintenance of effective cardiac pacing therefore depends on reliable evoked response sensing. False capture detection can result from oversensing of cardiac signals or non-cardiac noise, such as electromagnetic interference or nearby skeletal muscle depolarizations. False capture detections may result in prolonged episodes of subthreshold cardiac pacing that is ineffective in maintaining a base heart rate, which can be detrimental to the patient's health and even fatal. False loss of capture detections can result from undersensing of the evoked response. False loss of capture detections can trigger the delivery of unnecessary backup pacing pulses and pacing threshold searches. Increases in pacing pulse energy due to false loss of capture detections can lead to premature pacemaker battery depletion.
A major difficulty in sensing an evoked response arises from the polarization artifact that immediately follows a pacing pulse. Polarization at the electrode-tissue interface causes an afterpotential signal that can saturate sense amplifiers included in the cardiac pacing device and mask an evoked response signal. Typically, a blanking interval is applied to sense amplifiers during and immediately following a pacing pulse to prevent saturation of the amplifiers. The polarization artifact may diminish during the blanking interval, however, it may still interfere with evoked response sensing. Low-polarization electrodes have been proposed for reducing the polarization artifact. See for example U.S. Pat. No. 4,502,492, issued to Bornzin, or U.S. Pat. No. 6,430,448, issued to Chitre, et al.
Improved methods for performing capture verification based on evoked response sensing have been proposed. Such methods may include special hardware circuitry or special software signal processing methods that reduce or eliminate the problem of polarization artifact. Reference is made to commonly assigned U.S. Pat. No. 6,134,473, issued to Hemming et al. and U.S. Pat. Application No. 20020116031 issued to Vonk, incorporated herein by reference in their entirety. Capture verification methods indicate when capture or loss of capture occurs, but generally do not indicate the confidence or reliability of the detection based on the quality of the evoked response signal. A process to verify capture that involves assessing the reliability of a chosen parameter of an evoked signal as a reliable indication of the response is disclosed in U.S. Pat. No. 5,855,594, issued to Olive, et al, incorporated herein by reference. The reliability of a sensed parameter will depend largely on the quality of the sensed signal. A sensed parameter that is not reliable for evoked response sensing on one sensing electrode pair may be reliable using another sensing electrode pair.
Cardiac pacing leads are often configured having a tip electrode and a ring electrode spaced somewhat back from the tip electrode. Bipolar pacing between the tip and ring electrode is often preferred over unipolar pacing between the tip electrode and pacing device housing because bipolar pacing thresholds can be lower than unipolar. Bipolar sensing of intrinsic cardiac P-waves and/or R-waves for monitoring a patient's intrinsic heart rate to determine the need for pacing can also be preferred over unipolar sensing because bipolar sensing can result in a better signal-to-noise ratio. Bipolar sensing of intrinsic signals can be improved further by shortening the spacing between a tip electrode and a ring electrode to reduce oversensing of far-field cardiac signals or non-cardiac noise. However, a shorter tip-to-ring spacing can make bipolar evoked response sensing more difficult because the evoked response signal may have already passed the ring electrode by the time the polarization signal has diminished.
Selection of separate sensing electrodes for sensing the evoked response, different than the electrode pair used for delivering the pacing pulse, can reduce or eliminate polarization artifact problems. Sensing a far-field signal related to an evoked response, as opposed to the near-field evoked response signal, or sensing a conducted polarization away from the pacing site has been proposed. See for example, U.S. Pat. No. 5,324,310 issued to Greeninger, U.S. Pat. No. 5,222,493 issued to Sholder, U.S. Pat. No. 5,331,966 issued to Bennett et al., U.S. Pat. No. 6,434,428 issued to Sloman, et al., and U.S. Pat. App. No. 20010049543, issued to Kroll. In pacing systems having alternative sensing electrodes available and programmable selection of sensing electrodes, alternate sensing electrodes may be selected if capture detection is inadequate using a default evoked response sensing electrode pair. However, manual selection of an optimal evoked response sensing electrode pair can be a time-consuming process and can be “hit-or-miss” since only capture or loss of capture information is generally provided without information regarding the evoked response signal quality.
Automatic switching of electrode polarity in cardiac pacing devices has also been proposed. Electrode switching/selection is generally disclosed in U.S. Pat. No. 4,628,934 issued to Pohndorf et al., and U.S. Pat. No. 6,085,118 issued to Hirschberg et al., both of which are incorporated herein by reference. Automatic switching between unipolar and bipolar operation during each pacer cycle to optimize the choice of unipolar and bipolar operation for given pacemaker events is generally disclosed in U.S. Pat. No. 4,549,548 issued to Wittkampf, et al., incorporated herein by reference. In the '428 patent cited above, switching between bipolar sensing in the atrium and unipolar sensing during a far-field interval window for detecting far-field R-waves for verification of atrial capture is generally disclosed.
However, automatic switching/selection of electrodes does not necessarily ensure that an optimal evoked response sensing electrode configuration will be selected. When multiple electrodes are available, evoked response sensing may be more reliable along one sensing vector than another. A method for automatically determining an optimal electrode configuration for measuring a metabolic parameter such as minute volume used for metabolic rate responsive pacemakers is generally disclosed in U.S. Pat. No. 5,707,398, issued to Lu. This method, however, does not address optimal electrode determination of evoked response sensing. What is needed therefore, is a method for automatically selecting an optimal evoked response sensing vector based on an evaluation of the evoked response signal quality.